The preparation of hydrocarbons from an H.sub.2 /CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst is known in the literature as the Fischer-Tropsch hydrocarbon synthesis process. Catalysts frequently used for the purpose contain one or more metals of the iron group together with one or more promoters and a carrier material. Suitable techniques for the preparation of these catalysts are such known techniques as precipitation, impregnation, kneading and melting. The products that can be prepared by using these catalysts usually have a very wide range of molecular weight distribution and, in addition to branched and unbranched paraffins, often contain considerable amounts of olefins and oxygen-containing organic compounds. Usually only a minor portion of the products obatined is made up of middle distillates. Of these middle distillates not only the yield but also the pour point is unsatisfactory. Therefore the direct conversion of H.sub.2 /CO mixtures according to Fischer-Tropsch is not a very attractive route for the production of middle distillates on a technical scale.
In this patent application "middle distillates" should be taken to be hydrocarbon mixtures whose boiling range corresponds substantially with that of the kerosene and gas oil fractions obtained in the conventional atmospheric distillation of crude mineral oil. The middle distillate range lies substantially between about 150.degree. and 360.degree. C.
Recently a class of Fischer-Tropsch catalysts was found which have the property of yielding a product in which only very minor amounts of olefins and oxygen-containing organic compounds occur and which consists virtually completely of unbranched paraffins, a considerable portion of which paraffins boils above the middle distillate range. It has been found that the high-boiling part of this product can be converted in high yield into middle distillates by means of hydrocracking. As feedstock for the hydrocracking at least the part of the product is chosen whose initial boiling point lies above the final boiling point of the heaviest middle distillate desired as end product. The hydrocracking, which is characterized by a very low hydrogen consumption, yields middle distillates with a considerably better pour point than those obtained in the direct conversion of an H.sub.2 /CO mixture according to Fischer-Tropsch.
The Fischer-Tropsch catalysts belonging to the above-mentioned class contain silica, alumina or silica-alumina as carrier material and cobalt together with zirconium, titanium, chromium and/or ruthenium as catalytically active metals in such quantities that the catalysts comprise 3-60 parts by weight of cobalt and 0.1-100 parts by weight of zirconium, titanium, chromium, and/or ruthenium per 100 parts by weight of carrier material. The catalysts are prepared by depositing the metals involved on the carrier material by kneading and/or impregnation. For further information on the preparation of these catalysts by kneading and/or impregnation reference may be made to U.S. Pat. No. 4,522,939 (Minderhoud et al).